Copying machines, and image forming systems using image reading and forming apparatuses have conventionally been used. Such a machine or system is designed to obtain an exact copy of an original. For this purpose, it is important to adjust the density when read image data is output by the image forming apparatus.
For example, a known copying machine corrects the image density by printing a specific pattern on a printing sheet by a printer, reading the pattern by an image reader, comparing the actually read value with a reference value, and correcting the density.
Density correction is, however, effective only when an image is formed on the basis of image data obtained by using the image reading apparatus, i.e., when the copying machine is used. This density correction is not suitable when an image is formed by using image data acquired without using the image reading apparatus or printing data such as PDL (Page Description Language), e.g., when a network-connectable copying machine is used as a printer from a computer on a network.